The present invention relates to graphics plotters including a pinch roller drive on a drum and, more particularly, to an improved pinch roller assembly whereby the pinch roller is mounted to a carriage slideably mounted on a supplemental track below the main track for easy translational movement and first bias means are provided for gently urging the carriage into alignment and second bias means are provided for urging the pinch roller down and the carriage to grip the supplemental track with sufficient force to prevent further translational movement. A releasable bar is pivotally mounted to the carriage for selectively holding the pinch roller off the drum and to release the gripping force on the supplemental track during translational adjustment of the assembly.
Graphics plotters employing a drum with pinch roller drive are well known in the art. In such devices, a rotatable drum is bi-directionally movable by a motor drive assembly to move a strip of paper or other writing medium beneath a pen or other writing device. The pen is mounted to a pen carriage assembly moving at right angles to the strip of paper's movement across the drum in response to a signal. Between the movements of the pen carriage and the drum, the two right angular directions of movement required are affected such that the plot is created on the paper. To move the paper with the drum, a pair of pinch rollers are provided with one adjacent each edge of the paper urging the paper against the surface of the drum and rolling along the paper at the point of contact with the drum as the drum is turned so that the paper is moved along with the drum. In order for no tangential forces to be created on the paper tending to buckle it or move it out of its proper alignment with the drum, the rotational axes of the pinch rollers and the drum must be kept in perfect parallel alignment. Additionally, since the width of the paper employed may change, the pinch rollers are usually made adjustable as to their position along the surface of the drum. Unfortunately, to date, this has created two mutually exclusive conditions. To maintain the pinch rollers in their proper alignment, the carriage mounting to the carriage track has been with such close tolerance and with such biasing force that translational movement of the pinch roller carriage has been almost impossible and accomplishable only through the application of an excessively high longitudinal translational force thereagainst.
Wherefore, it is the object of the present invention to provide a pinch roller assembly which automatically attains and maintains the desired positional alignment between the pinch rollers and the drum while, at the same time, providing for easy translational adjustment of the rollers with a minimum of force.